1. Field of the Invention
The present invention relates to a color developer for a pressure-sensitive recording paper. More particularly, the present invention relates to a color developer composed of an acid-treated clay mineral, which is capable of forming an image having a high density and a good light resistance by color reaction with a leuco dye or the like.
2. Description of the Related Art
Color reaction of transfer of electrons between a colorless compound of an organic dye having an electron-donating property and a color developer as an electron acceptor is generally utilized for pressure-sensitive recording papers. Known color developers (color formers) are roughly divided into an inorganic acid such as a clay mineral, for example, silica, or an acid-treated product thereof, a phenolic resin formed by reaction between a phenol and formaldehyde, and a zinc salt of an aromatic hydroxycarboxylic acid.
Many proposals have been made on color developers composed of acid-treated clay minerals. For example, Japanese Examined Patent Publication No. 41-7622 proposes a color former for a non-carbon recording paper, which is obtained by treating acid clay or a similar clay with a mineral acid to elute alumina, iron and chlorine components soluble in the acid and which has a specific surface area of at least 200 m.sup.2 /g. Furthermore, Japanese Examined Patent Publication No. 44-2188 teaches that the secondary coloring performance (K.sub.2) of a dioctahedral type montmorillonite clay mineral to Benzoyl Leucomethylene Blue is peculiar to the production place or deposit position, and that if a clay mineral having a secondary coloring performance (K.sub.2) exceeding a certain reference value is selected and acid-treated so that the specific surface area is at least 180 m.sup.2 /g, there can be obtained a color former having an excellent coloring effect to both of a primary color-forming dye and a secondary color-forming dye.
Furthermore, Japanese Examined Patent Publication No. 63-15158 discloses a process for the preparation of a color former for a pressure-sensitive recording paper, which comprises acid-treating a clay mineral having a layer structure consisting of tetrahederons of silica so that the SiO.sub.2 content on the dry base is 82 to 96.5% by weight and the diffraction pattern based on the crystal of the layer structure by the X-ray diffractometry and the diffraction pattern based on the crystal of the layer structure by the electron beam diffractometry are not substantially manifested, and introducing a magnesium component and/or an aluminum component in the acid-treated product so that the diffraction pattern based on the crystal of the layer structure by the electron beam diffractometry is manifested again.